The Ka-band of the electromagnetic spectrum is the radio frequency band between about 17 and 36 GHz. This upper portion of the microwave range is used primarily for satellite communication. Many two-way satellite communication systems transmit and receive data in the Ka-band. However, other two-way satellite communication systems transmit and receive data in various other bands such as the C-band (3.7–6.4 GHz) and the Ku-band (11–15 GHz), for example. Future systems may use higher frequencies (e.g., 60 GHz).
Modulation and upconversion are essential methods used in wireless communication systems, including two-way satellite communication systems. Upconversion is the translation of a signal's frequency from baseband, or the original frequency before modulation, to a higher frequency. The signal is then transmitted at this higher frequency. Upconversion is done because most antennas can only receive signals that have short wavelengths. Frequency is the inverse of wavelength. Therefore, the higher the frequency a signal has, the shorter its wavelength.
Modulation is a method used to transmit and receive digital signals. By varying the phase of the transmitted signal, for example, information can be conveyed. This type of modulation is called phase-shift keying (PSK). There are several schemes that can be used to accomplish PSK. The simplest method uses only two signal phases: 0 degrees and 180 degrees. The digital signal is broken up time wise into individual bits (binary digits—zeros and ones). The state of each bit is determined according to the state of the preceding bit. If the phase of the wave does not change, then the signal state stays the same (low or high). If the phase of the wave changes by 180 degrees—that is, if the phase reverses—then the signal state changes (from low to high, or from high to low). Because there are two possible wave phases, this form of PSK is sometimes called Binary Phase Shift Keying (BPSK).
A more complex form of PSK is called Quadrature Phase Shift Keying (QPSK). QPSK modulation employs four wave phases and allows binary data to be transmitted at a faster rate per phase change than is possible with BPSK modulation. In QPSK modulation, the signal to be transmitted is first separated into two signals: the In-phase (I) signal and the Quadrature (Q) signal. The I and Q signals are orthogonal, or 90 degrees out of phase. Thus, they are totally independent and do not interfere with each other. Each signal can then be phase shifted independently. Both the I and Q signals have two possible phase states. Combining the possible states for the I and Q signals results in four total possible states. Each state can then represent two bits. Thus, twice the information can be conveyed using QPSK modulation instead of BPSK modulation. For this reason, QPSK modulation is used in many two-way satellite communication systems.
For any two-way satellite communication system using a QPSK modulator and upconverter, there are a number of competing design goals. First, the system should have low phase noise. Phase noise is a result of rapid, short-term, random fluctuations in the phase of a wave and is caused by instabilities found in oscillators.
Low levels of spurious emissions (also called spurious noise) are also desirable. Spurious emissions are emissions on a frequency or frequencies which are outside the necessary bandwidth of the transmitting signal, but still within the band. These emissions may be reduced without affecting the corresponding transmission of information. Spurious emissions include intermodulation distortion and harmonic distortion. Intermodulation distortion is a result of emissions on the sum and difference frequencies of the fundamental frequencies of the transmitted signal. Harmonic distortion is a result of emissions on frequencies that are not present in the input signal. Both distortions are caused by nonlinearities in the devices used to modulate the signals.
Another design goal is that there should also be a large frequency hopping range. Frequency hopping is a modulation technique that involves the repeated switching of frequencies during transmission. Frequency synthesizers generate the frequencies that are to be hopped to. A small frequency settling time, or the time it takes for the frequency synthesizer to lock into the new frequency, is also desirable. In the case of an example Ka-band two-way satellite communication system, there are four 125 MHz bands over which the frequency synthesizer must operate. Each band is partitioned into a number of channels. In one example, a class A two-way satellite communication system is defined to have 175 channels per band. In another example, a class B two-way satellite communication system is defined to have 35 channels per band. The frequency synthesizer must preferably be able to hop to the center frequency of each channel within a few nanoseconds.
A very fine frequency accuracy and step size is preferably required to compensate for, or correct, the Doppler effect. The Doppler effect refers to the phenomenon of a signal's frequency being affected by the relative motion of the transmitter and receiver. When the signal source is approaching the observer, for example, the signal's frequency increases. Because satellites are constantly moving, the modulator and upconverter must preferably compensate for the Doppler effect. The Doppler frequency may range from −160 Hz to +160 Hz in two-way geostationary satellite communication.
Finally, there should be small amplitude and group delay variation across the hopping band. Amplitude variation happens when the signal has different amplitudes across the band. Group delay is the rate of change of the total phase shift with respect to angular frequency through a transmission medium. It is desirable to maintain both a constant amplitude and group delay across the hopping band.
Currently, the Ka-band upconversion entails a multi-stage conversion process. First, baseband QPSK I,Q streams are modulated and then upconverted to an Intermediate Frequency (IF) in the L-band range (e.g., 1.7–2.2 GHz). This conversion is performed by in an Indoor Unit (IDU). The signal is then upconverted again and amplified to 29.5 to 30.0 GHz in an Outdoor Unit (ODU) located at the terminal's antenna. The upconversion is then complete and the signal is ready for transmission. To meet frequency accuracy requirements, the Local Oscillators (LO's) in all the upconversion stages can be phase locked to a single reference (e.g., a reference locked to the stable satellite payload oscillator available in the satellite downlink signal).
The IDU and the ODU are connected via some type of cable, for example RG-6. This type of cable performs well and has relatively small losses (10–15 dB per 100 feet) at frequencies of 1.7–2.2 GHz. In addition, RG-6 cable is easy to procure because this IF range is a common range used currently with digital satellite television set-tops. Thus, there are many RG-6 suppliers. However, other kinds of cables could also possibly be used to connect the IDU and the ODU.
Because of the various competing design goals mentioned above, there are many tradeoffs that are made in the IDU modulator and IF upconverter design. For example, single analog upconversion from baseband to IF can achieve low spurious and phase noise, large hopping range, and small amplitude and delay variation. However, these gains are achieved at the expense of a large frequency settling time and large step sizes. Direct-Digital Synthesis (DDS) can be used in these analog synthesizers to improve settling time and decrease step size but can require costly filtering to achieve low spurious noise. Heterodyne architectures (e.g., double analog upconverters) can be used to reduce the spurious noise. However, such architectures require complex analog bandpass filtering that risks increasing amplitude and group delay variation.
An alternate approach to meet the design goals above is to use an all-digital upconverter to accomplish the upconversion from baseband to IF. This, however, forces the Digital-to-Analog Converter (DAC) to operate at a very high sampling rate (e.g., greater than 1.7–2.2 GHz). DACs that operate at these high sample rates are currently difficult to design and are not cost-effective for most applications.
Digital upconversion can be used in conjunction with analog IF upconversion to achieve fast hopping and small step size over a limited bandwidth. Digital process technologies (CMOS) enable current designs of Numerically Control Oscillators (NCOs) to economically run at 200–400 MHz clock frequencies to achieve frequency hopping bandwidths of 50–100 MHz. When combined with analog upconversion, however, there can be serious spurious emission problems. For example, a digital I/Q upconversion to a center frequency of fd=10 MHz requiring analog upconversion to 1.7 GHz utilizes an analog LO of fVCO,IF=1.71 GHz or 1.69 GHz. DAC and analog mixer nonlinearities and unbalances induce spurious noise (intermodulation distortion) at IF at frequencies of ±nfVCO,IF±mfd for integers m and n. It is difficult to sufficiently filter (reject) intermodulation products at multiples of 10 MHz from the desired carrier frequency.
Another problem with using digital upconversion in conjunction with analog IF upconversion has to do with the compensation of the Doppler effect. A prior solution included compensating for the Doppler effect in the analog IF upconversion stage by slightly varying fVCO,IF. This is difficult and costly because Doppler compensation requires very fine frequency accuracy and very fine step size. Varying fVCO,IF also induces spurious noise at frequencies in adjacent channels. Adjacent Channel Emissions (ACE) specifications are stringent at large offsets from the desired carrier frequency and more lenient close to the signal bandwidth. Thus, the increased spurious noise in adjacent channels due to Doppler compensation in the analog IF upconversion stage results in additional necessary filtering that is difficult and expensive. Thus, in a combined digital/analog modulator and upconverter used in the IDU, there is a need in the art for a method and system that compensate for the Doppler effect while avoiding out of channel spurious noise and not requiring an analog IF frequency synthesizer with very fine frequency accuracy and very fine step size.
In between each of the four 125 MHz bands, there is a guard band. A guard band is a frequency band that is deliberately left vacant between two bands to provide a margin of safety against mutual interference. In many two-way satellite communication systems, the guard band's width is not a multiple of the channel widths. This poses a problem in the design of the analog IF frequency synthesizer. In tuning to a particular channel in one band and then hopping to a different channel in another band, the frequency synthesizer skips over the guard band. A traditional frequency synthesizer needs a small step size (e.g., 2.5 kHz) to accomplish this. This results in an undesirably high phase noise. Thus, in a digital combined with analog modulator and upconverter used in the IDU, there is a need in the art for a method and system that allow an analog IF frequency synthesizer to tune to different channels while skipping over the guard bands with a large enough step size that will maintain the phase noise within acceptable levels.
As used hereafter and in the appended claims, the term “two-way satellite communication systems” will be used to refer expansively to all possible two-way satellite communication systems and other wireless communication applications in any band where frequency hopping with or without Doppler compensation is desired.